Competition between wheat and associated weeds at different nitrogen rates.
Field trials were conducted over 2 consecutive winters to evaluate the effects on weed competition in wheat cv. WH-329 on sandy loam of 0-120 kg/ha N (50% supplied at sowing and 25% at each of the first and second irrigations) and post-em. treatment at 31 or 36 d after sowing (DAS) with 1.0 kg/ha isoproturon ± 0.5 kg 2,4-D or 0.75 kg isoproturon + 0.1% Teepol surfactant. Isoproturon + 2,4-D resulted in absolute weed control, whereas the other herbicides increased the DW ratio of wheat:weeds from 9.5-11.1 to 23.1-27.1 and 4.5-4.9 to 12.5-16.0 in the 2 years. N concn had no effect on the ratio or on that of wheat:wild oat [Avena spp.] which varied from 0.9 to 1.6 at harvest and was generally higher at low Avena densities (45 plants/m2). The DW of broadleaved weeds (mainly Chenopodium album, Anagallis arvensis, Melilotus alba and Vicia sativa) increased to a max. of 40-60 g/m2 at 90 DAS, whereas that of Avena rose sharply at 60 DAS to 250-400 g at harvest. In wheat the concn of N was correlated with the height and number of shoots, and yield with ear length and number of spikelets/spike.